1. Field of the Invention
The invention described herein relates to the field of superconductivity, and more specifically relates to circuits and techniques for implementing digital quadrature mixers using Josephson junctions.
2. Related Art
Josephson junctions are quantum-mechanical circuit elements of superconducting devices. The Josephson effect in particular results from two superconductors acting to preserve long-range order across a barrier, such as an insulating barrier. With a thin enough barrier, the phase of the electron wave function in one superconductor maintains a fixed relationship with the phase of the wave function in another superconductor. This linking up of phases is called phase coherence.
A Josephson junction is the interface between two superconducting materials separated by a non-superconducting barrier. A current may flow freely within the superconductors but the barrier prevents the current from flowing freely between them. However, a supercurrent may tunnel through the barrier depending on the quantum phase of the superconductors. The amount of supercurrent that may tunnel through the barriers is restricted by the size and substance of the barrier. The maximum value the supercurrent may obtain is called a critical current of the Josephson junction.
Josephson junctions have two basic electrical properties. The first is that the junctions have inductive reactance. That is, similar to inductors, the voltage difference across the junction is related to the time rate of change of the current. The second is that a constant voltage across the junction will produce an oscillating current through the barrier, and vice versa. Thus, Josephson junctions convert a direct current voltage to an alternating current.
A family of logic/memory devices were proposed using Josephson junctions the IEEE Transactions on Applied Superconductivity, Volume 1, Number 1, March 1991, by K. K. Likharev and V. K. Semenov in an article entitled, RSFQ Logic/Memory Family: A New Josephson junction Technology For Sub-Terahertz-Clock-Frequency Digital Systems. That article is hereby incorporated by reference in its entirety into specification of this application.
RSFQ circuits are widely recognized as the fastest digital circuits in any electronic technology, and this is also true of RSFQ digital mixers. The digital mixers described in the present invention constitute the first practical circuits for the implementation of digital mixers in a complete RSFQ digital receiver system, and have been demonstrated for clock speeds up to 40 GHz.
Prior art attempts at producing digital mixers in the superconducting domain required synchronism between an incoming signal and a reference signal.